The present invention relates to a ball screw and, in particular, to an improved ball screw of an end cap type which includes a ball circulation passage in the end portion of a ball screw nut.
A conventional ball screw device of an end cap type, for example, is shown in FIG. 5. In the conventional ball screw device, a screw shaft 1 including a spiral groove 1a on the outer peripheral surface thereof is threadedly engaged with a cylindrical ball screw nut 2 including in the inner peripheral surface thereof a spiral groove 2a corresponding to the spiral groove 1a of the screw shaft 1 through balls 3 which can be rolled within the two mutually opposing spiral grooves 1a, 2a. The ball nut 2 comprises a nut main body 4 and two disk-shaped ball circulation members (end caps) 5 removably connected to the two end faces of the nut main body 4. The nut main body 4 includes in the thicker portion thereof a ball return passage 6 which consists of a through hole extending the axial direction thereof. The ball circulation member 5 includes, in the end face to be connected with the nut main body 4, a curved passage 7 which connects the two mutually opposing spiral grooves 1a, 2a with the ball return passage 6.
When the screw shaft 1 and ball screw nut 2 are rotated relatively to each other, then the balls 3 of steel roll and advance within the two mutually opposing ball screw grooves 1a, 2a of the screw shaft 1 and screw nut 2, and pass through the curved passage 7 formed in the ball circulation member 5 of the nut main body 4 end face and through the ball return passage 6 formed in the nut main body 4 to return to their original position, where the circulation movements will be repeated.
The curved passage 7, which is formed in the ball circulation member 5 of the conventional ball screw of an end cap type, has a complicated and delicate shape in three dimensions. If the curved passage 7 is formed by machine working, then it is inevitable that the working costs thereof will be expensive. Therefore, in the conventional ball screw of an end cap type, it is generally known to use a ball circulation member 5 of synthetic resin by use of injection forming which is able to mass produce parts each having a complicated shape at low costs.
However, the ball circulation member 5 of synthetic resin is relatively low in rigidity. For this reason, when the balls 3 circulate, the balls 3 collide with the inner surface of the curved passage 7 repeatedly to thereby cause the ball circulation member 5 to vibrate in an elastic deformation mode, which produces an unfavorably large noise in uses such as a business machine, a measuring instrument and the like requiring a low noise.
Also, when the ball screw nut 2 is moved back and forth unrotationally by rotationally driving the shaft 1, if the ball screw nut 2 overruns and collides with other parts or the like, then the ball circulation members 5 on the end face of the nut 2 can be easily broken. To solve this, it can be expected that the ball circulation members 5 are formed of a material having a relatively higher rigidity such as a metal material and the like. However, in this case, it is expensive to form the complicated curved passage, which results in an expensive ball screw of an end cap type.